What is Magnetic Resonance Imaging (MRI)?

Magnetic resonance imaging (MRI) method using a strong magnetic field (B0 field) and gradient fields to localize bursts of radiofrequency signals coming from a system of spins consisting of reorienting hydrogen H nuclei (protons) after they have been disturbed by radiofrequency RF pulses. MR imaging produces high resolution, high contrast two-dimensional image slices of arbitrary orientation (see Fig.1), but it is also a true volume imaging technique and three-dimensional volumes can be measured directly.

MR imaging is furthermore capable of quantifying velocity and higher order moments of motion and thus quantitating blood flow. Applications of MR imaging have steadily widened over the last decade. Currently it is the preferred cross-sectional imaging modality in most diseases of the brain and spine and has attained major importance in imaging diseases of the musculoskeletal system. MR imaging in the head and neck and pelvis has attained a substantial level of clinical use, and its applications in the abdomen, kidneys and chest are rapidly increasing with the advent of ultrafast MR imaging techniques.

MR imaging makes use of the NMR phenomenon, i. e. the fact that many nuclei exhibit a property called spin. These spins are orientated in an external magnetic field. External radiofrequency pulses disturb their orientated state and make them absorb energy, which is subsequently reradiated. The intensity of the reradiated signal is dependent on the radiating tissue and the pulse sequence used to disturb the spins. Since the NMR phenomenon has many contrast mechanisms, MR imaging is very rich in contrast. It is mainly determined by T1 relaxation and T2 relaxation processes, but other parameters such as the density of mobile protons (proton density), susceptibility effects, magnetization transfer MT , diffusion and flow effects can also be made relevant contrast determining parameters. MR imaging requires spatial localization of the NMR-signal which is accomplished by using additional magnetic gradient fields. As a result, the signal behaviour can be observed in small volume elements (voxels). Image data reconstruction is currently performed most frequently with the technique called Fourier transform imaging.

The signal measured in MR imaging is weak and can only be observed because of the very large number of proton spins in human tissue. The major concern in imaging is, therefore, to obtain an adequate signal to noise ratio SNR in the images. This can be accomplished in several ways. First, an increase in strength of the main magnetic field increases the SNR almost quadratically at lower field strengths, linearly at higher field strengths. Averaging multiple measurements also improves SNR but it only increases with the square root of time. A further widely used strategy to improve the SNR is the use of a local coil (surface coil). The smaller the region of interest to be imaged, the smaller the local coil and the better the SNR.

MR imaging is accomplished using an MR imager, which is a complicated system consisting of a magnet, additional magnetic gradient fields, a radiofrequency emitter and receiver, and computer systems for control and image reconstruction.

Typical sagittal MR image of the head. This image is historical in that it was taken from the head of Prof. R.R. Ernst in 1985 (see Ernst angle (I)): "image of the brain which made it possible for you to see this image". Personal permission.

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